1. Field of the Invention
The present invention relates to a group-3 nitride semiconductor device (referred to as device hereinbelow), and, in particular a nitride semiconductor light emitting device and a manufacturing method thereof.
2. Background Art
Extensive research is now underway on gallium nitride (referred to as GaN hereinbelow) and related compounds as a material system for a shortwave light emitting device, in particular, a shortwave semiconductor laser. A GaN-based semiconductor laser device is manufactured by successively depositing semiconductor single-crystal layers such as (Al.sub.x Ga.sub.1-x).sub.1-y In.sub.y N (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1) on a crystal substrate.
A metal organic chemical vapor deposition method (abbreviated as MOCVD hereinbelow) is generally used to produce such a single-crystal layer. In this method, source gases containing trimethyl gallium (abbreviated as TMG hereinbelow) as a group-3 precursor material and ammonia (NH.sub.3) as a group-5 precursor material are introduced into a reactor to react at a temperature within the range of 900-1000.degree. C., thereby depositing compound crystals on the substrate. A multi-layer structure comprising various compounds can be obtained by changing the ratio of the precursors fed into the reactor to grow many different layers on the substrate.
If the deposited single-crystal layer has many penetrating defects, the light emitting performance of the device is deteriorated substantially. Such defect is called threading dislocation, which is a linearly extending defect that penetrates the crystal layer along the growth direction. Since a threading dislocation acts as a non-radiative recombination center for carriers, a semiconductor light emitting device comprising a layer with many dislocations suffers from poor luminous efficiency. The above mentioned defect is generated due to crystal misfit strain at an interface between the substrate and an overlying layer formed thereon. Attempts to reduce the effect of the misfit at the interface have been made by choosing substrate materials having similar crystal structure, lattice constant, and thermal expansion coefficient to those of GaN-based crystal.
A material, which meets the above requirements and has good compatibility with a substrate, is a semiconductor crystal itself. However, as for group-3 nitride semiconductors (Al.sub.x Ga.sub.1-x).sub.1-y In.sub.y N (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1), it is inevitable to use dissimilar materials such as sapphire or the like, because there is no nitride semiconductor bulk crystal which is most suitable for a substrate. Sapphire has a lattice constant different from that of GaN by about 14%.
One approach, known as the two-step-growth method, was proposed to accommodate the misfit at the interface between a sapphire substrate and a Gan-based single-crystal layer grown thereon to reduce the generation of crystal defects in the GaN-based single-crystal layer. This method comprises the steps of forming a lower-temperature buffer layer consisting of aluminum nitride (AlN) on the sapphire substrate at a lower temperature within the range of 400-600.degree. C., and then forming a GaN single-crystal layer over the lower-temperature buffer layer. However, the above method has not been completely successful in reducing the generation of such defects that pass through the GaN single-crystal layer.
Generally, as a dislocation in semiconductor crystals acts as a non-radiative recombination center and is substantially responsible for degrading the light emitting characteristics of light emitting devices such as light emitting diodes and semiconductor lasers, it is desirable that the crystals in these devices do not includes any dislocations. Therefore, research is now underway toward reduction of threading dislocations.
A main object of the invention is to provide a nitride semiconductor light emitting device having good luminescent characteristics.
Another object of the invention is to provide a method for manufacturing the above nitride semiconductor light emitting device whereby the generation of defects passing through a single-crystal layer formed on a substrate can be reduced.